1. Field of the Invention:
This invention relates to an amplifying FET (Field Effect Transistor) semiconductor element equipped with a feedback circuit for a monolithic IC. More particularly the invention relates to an amplifying feedback-type FET semiconductor element having a wide-band frequency characteristic and adapted to be used in communications systems such as radar.
2. Description of the Related Art:
In modern communications systems, GaAs FETs are widely used as semiconductor elements for microwave amplifiers which are low-noise and high-gain.
Especially in recent years, this kind of wide-band amplifier using FETs has been further developed into a GaAs monolithic IC form. In such development, since resistors can be easily produced from GaAs, a wide-band amplifier can be accomplished by providing the GaAs FET with a negative feedback by the resistor.
FIG. 5 of the accompanying drawings is a circuit diagram showing a typical conventional feedback-type field-effect-transistor amplifier (hereinafter called "feedback-type FET amplifier") disclosed in a thesis publication MW80-80 entitled "GaAs Monolithic IC Ultrawide-band Low-noise Amplifier" (by Nishiuma et al.) issued from The Institute of Electronics, Information and Communication Engineers.
In FIG. 5, 1 designates a field effect transistor (hereinafter called "FET") which has a gate terminal 2, a drain terminal 3 and a source terminal 4. The source terminal 4 is connected to the ground. The gate and drain terminals 2, 3 of the FET 1 are connected by a feedback circuit 8 composed of a resistor 5, a capacitor 6 and connecting transmission lines 7.
Thus this feedback circuit 8, as shown in FIG. 5, includes passive elements, i.e. the resistor 5 and the capacitor 6, and the connecting transmission lines 7.
In this circuit, the impedance Z.sub.f of the feed-back circuit 8 is expressed by the following equation: EQU Z.sub.f =R.sub.f +j (.omega.L.sub.f -1/.omega.C.sub.f) . . . (1)
where R.sub.f is the value of the resistor 5, C.sub.f is the value of the capacitor 6, and L.sub.f is the value of the inductance of the connecting transmission lines 7.
In a feedback-type FET amplifier using such FET semiconductor elements, if the value C.sub.f of the capacitor 6 is determined to be sufficiently large and also if the value L.sub.f of the connecting transmission line 7 is determined to be sufficiently small, the second term j(.omega.L.sub.f -1/.omega.C.sub.f) on the right side of the equation (1) above is negligible, compared with the first term R.sub.f on the right side, in a desired frequency band. Therefore if the impedance Z.sub.f of the feedback circuit 8 is appropriated to the value R.sub.f of the resistor 5 (Z.sub.f =R.sub.f) and if a most suitable value is selected for the value R.sub.f of the resistor 5, this amplifier can have good input and output reflection characteristics over a wide frequency band and also a frequency characteristic showing a flat gain over a wide frequency band.
FIG. 6 is a chip pattern diagram showing one example of the conventional amplifying feedback-type FET semiconductor element of FIG. 5.
In this conventional amplifying feedback-type FET semiconductor element, as shown in FIG. 6, if the gate width of the FET 1 is increased in an effort to obtain a high output, the distance between the drain and gate terminals 3, 2 of the FET 1 is increased so that the connecting transmission lines 7 of the feedback circuit 8 become elongated. Consequently, the inductance L.sub.f of the connecting transmission lines 7 cannot be neglected so that the impedance Z.sub.f of the feedback circuit 8 depends on the frequency. Therefore, with the frequency characteristic of the conventional amplifier, it is difficult to obtain a flat gain over a wide frequency band.